The instant invention relates to an adjustable viscous shear coupling which comprises a housing containing a viscous fluid, a shaft extending into the housing, external disks non-rotatably connected to the housing, internal disks non-rotatably connected to the shaft and extending between the external disks, and an axially movable adjusting piston to change the gap width between the disks, and in which no unregulated steep increase in torque occurs in any operational state of the viscous shear coupling due to a rise of internal pressure.
Adjustable viscous shear couplings are used in the drives of all-wheel drive vehicles, especially in vehicles with so-called permanent all-wheel drive. Such couplings are used mainly to establish the drive connection between the two driven axles, but sometimes also in lieu of intra-axle differentials.
Adjustable viscous shear couplings are used when a significant torque must be transmitted not only during off-the-road driving, but also during high speed road driving by a high performance vehicle, e.g., in sports cars, where it is desired that traction by all four wheels be utilized to accelerate the vehicle. In order to achieve this, a suitably sensitive adjustment of the transmitted torque is required and therefore an adjustable design for the viscous shear coupling is needed.
With the all-wheel drives normally used in off-the-road vehicles, a viscous shear coupling that is not adjustable through external intervention is adequate, because in such couplings heat and pressure rise with continuous slippage and, if the disks are designed accordingly, the so-called "hump" occurs. In this spontaneously occurring state of operation, disk pairs are brought to bear against each other so that mixed friction occurs leading to an uncontrolled and steep, although limited, increase of torque. It is limited because some slippage is still present even in this state of operation, so that the cooling action is greater than the heat loss occurring in the coupling, and effective overload-protection is achieved.
In the case of a regulated permanent all-wheel drive for high-performance vehicles to which the instant invention relates, however, conditions are basically different. The relationship between slippage and torque is generally adjusted through external modification of the distance between the disks so that the "hump" can be dispensed with, in fact must be dispensed with, because a spontaneously occurring process would interfere with the adjustment being made. At the same time, an adjustment range that is as great as possible is of course desired.
An adjustable viscous shear coupling in which the gap between the disks is adjusted by external means is known from AT-PS 384,086 (U.S. Pat. No. 4,844,219). Furthermore, it is known that in the viscous shear coupling of this patent, the adjustment range may be increased by filling the housing partially and by using a fluid of high viscosity. See, e.g., U.S. Pat. No. 4,942,951.
It is a disadvantage in such case, however, that the torque transmitted remains restricted as before when very little slippage occurs, and that no torque is transmitted if slippage tends towards zero. Furthermore, the automatic overload protection afforded by the "hump" does not apply, so that the coupling may be damaged due to overheating. Also, in the range of high torque forces, the adjustment range is also limited due to the fact that the disks come into contact with each other unevenly when the gap between them is drastically reduced, causing high disk wear. It is a further disadvantage that unavoidable leak losses which occur over the long term alter the characteristics of the coupling thus affecting operating safety.
A viscous shear coupling having friction linings on the disks is known from U.S. Pat. No. 4,022,084 (Pagdin et al.). However, the viscous shear coupling disclosed therein is a non-adjustable coupling which permits the "hump" to occur. The effect of the friction linings is only to increase further the torque transmitted in the "hump state," but without ever enabling the coupling to transmit torque without slippage, and therefore without wear.
It is therefore the object of the instant invention to develop further an adjustable and "hump-less" viscous shear coupling suitable for use in a high performance, four wheel drive vehicle in such a manner that a wide adjustment range which does not change over time is available even with very little slippage, and in which torque can be transmitted entirely without slippage even in continuous operation.